1. Field of the Invention
This invention relates to ionomers which are partially neutralized metal salts of ethylene/carboxylic acid copolymers which have copolymerized comonomer units of both certain monocarboxylic acids and certain dicarboxylic acids or their derivatives. The new ionomers have many useful superior properties compared with existing conventional ionomers. In particular, the ionomers are useful when blended with polyamides to form toughened multiphase thermoplastic compositions.
2. Description of Related Art
Copolymers which are dipolymers of ethylene and the unsaturated monocarboxylic acids methacrylic or acrylic acid only, are well known. Commercially such dipolymers typically contain at least about 75 weight percent and up to about 92 weight percent ethylene. They are used for molding, packaging and some adhesive applications. Examples of such ethylene/acid copolymers include those sold under the trade name, Nucrel.RTM., which are produced by E. I. du Pont de Nemours and Company. Such ethylene/acid copolymers are disclosed in U.S. Pat. No. 4,351,931 (Armitage), the parent disclosure of which was filed in June 1961.
Acid copolymers of this type, having monocarboxylic acid comonomers are also used to prepare ionomers, wherein the carboxylic acid units of the copolymers are partially neutralized with metal ions. Such `ionomers` are sold by E. I. du Pont de Nemours and Company under the trade name Surlyn.RTM.. These resins are thermoplastic in the melt. Partial neutralization is used because fully neutralized acid copolymers are known to, and are disclosed as having, intractable melts. Ionomers have separate utility and significantly different properties from the un-neutralized ethylene/carboxylic acid copolymer `precursors`. Ionomers were first disclosed in U.S. Pat. No. 3,264,272 (Rees). The ions form `ionic crosslink` bonds at low temperatures, but are sufficiently labile at melt temperatures, provided there remain some un-neutralized carboxylic acid groups, that they allow typical thermoplastic melt processability.
The above Armitage patent does not disclose use of acid comonomers other than acrylic and methacrylic acid, and in practice these monocarboxylic acids are the only commercially important acid comonomers in ethylene/carboxylic acid copolymers as well as in the derived metal ion-neutralized ionomers. However, other acid comonomers which polymerize with ethylene, and the resulting copolymers have been disclosed; for example, in the above-mentioned Rees patent. In this patent, copolymers containing 0.2 to 25 mole percent of dicarboxylic acid comonomers such as itaconic, maleic, fumaric acids, their monoalkyl esters such as methyl hydrogen maleate, and anhydrides such as maleic anhydride are disclosed. Copolymers containing ethylene and 6 weight percent itaconic acid, and copolymers with 6 weight percent maleic anhydride are exemplified as are ionomers derived therefrom. The disclosure gives examples of ethylene/dicarboxylic acid dipolymers neutralized with mono- and divalent metals. In the case of monovalent metal ions, melt flow of partially neutralized copolymers is disclosed as being very low and, with divalent metal ions, flow is zero in several instances. While the degree of neutralization is not specifically stated in that disclosure, the inference is that, unlike ethylene/monocarboxylic acid ionomers, those of dicarboxylic acid copolymers are intractable, or very nearly so.
The Rees disclosure indicates that the intractable nature of ionomers of ethylene/dicarboxylic acids and divalent metal ions is believed to be because the ionic bonds are so strong that, `they act like regular cross-linked polyolefins`. In a review of ionomers by R. W. Rees, Chapter 10A--Ionomers Thermoplastic Elastomers, Early Research--Surlyn.RTM. and Related Polymers--in `Thermoplastic Elastomers--A Comprehensive Review`, ed. N. Legge et al. Publ. Hanser 1987, data are provided, and it is stated that neutralization of dipolymers of ethylene and dicarboxylic acids gives low flow materials with monvalent sodium ions and with divalent ions such as zinc and strontium, the flow is zero. The degree of neutralization is not specifically stated. The reference also suggests that ethylene/dicarboxylic acid copolymer ionomers are not outstanding in any way compared with monocarboxylic acid copolymer ionomers.
No acid copolymers, or their derived ionomers, which are terpolymers (or even higher comonomer copolymers) having both monocarboxylic acid and dicarboxylic acid comonomers (or their derivatives) are specifically disclosed in Rees. Terpolymers with non-acid termonomers such as alkyl acrylates and methacrylates and their derived ionomers are, however, disclosed. Later, ionomers of this type, specifically, partially neutralized terpolymers of ethylene, methacrylic acid and n-butyl or iso-butyl acrylate were commercialized and are also available under the trade name Surlyn.RTM.. These terpolymer ionomers are commonly referred to as `soft` ionomers, and the dipolymer ionomers, without `softening` acrylate termonomers, as `hard` ionomers.
Terpolymers of ethylene with between about 50 and 65 weight percent methyl acrylate and a low level of an alkyl monoester of maleic acid, cross-linked with a crosslinking agent which acts on the half ester, is cured with the agent to produce an elastomer, sold under the trade name Vamac.RTM. by E. I. du Pont de Nemours and Company. In this case the crosslinking cure typically produces covalent bond crosslinks. Such cured (cross-linked) resins are elastomeric but not melt processible. Such terpolymers and cross-linked elastomers derived from them are described in U.S. Pat. No. 3,904,588 (Green).
The terpolymers of ethylene, (meth)acrylic acid and alkyl acrylates noted above, form a class of acid copolymers with lower modulus (hence `soft`) and good low temperature properties. A method of preparing these acid copolymers is disclosed in U.S. Pat. No.4,690,981. While, as noted, they are used as the precursor acid copolymer to form `soft` ionomers by partial neutralization with metal ions, these acid copolymers with acrylates also find utility in their own right. These acrylate termonomer terpolymer ionomers, whether neutralized with mono- or divalent metals have melt flows at least as good as dipolymer ionomers, thus providing ready processability.
Polyethylenes grafted with a low level of diacid or diacid derivatives, such as maleic acid and its anhydride respectively, are well known for use in adhesives, the maleic acid or anhydride graft being specifically used to enhance adhesivity. According to (REES Chapter 10A) however, with partial neutralization of high density polyethylene grafted with either mono- or dicarboxylic acids to form ionomers, no benefits in mechanical properties were seen, and the main property change was a large reduction in melt flow.
There is a need for ionomers which are thermoplastically processible in the way that conventional ionomers are, but which have improved properties, or which provide a new range of properties when compared with the well known ethylene/monocarboxylic acid/(alkyl acrylate) dipolymer (or terpolymer) ionomers.
In addition, the use of ionomers such as the ethylene methacrylic acid neutralized ionomers sold under the trademark SURLYN.RTM. as an impact modifier for a material such as nylon is known. See U.S. Pat. Nos. 3,845,163 or 4,174,358. Improvement to certain impact modifying properties was achieved through use of acrylate type co-monomers such as n-butyl acrylate but this approach as disclosed in U.S. Pat. No. 4,801,649 leads to a trade off in certain properties such as mechanical stiffness. Other nylon modifiers are also known such as those grafted olefin rubbers such as those disclosed in Epstein, U.S. Pat. No. 4,174,358. These compositions when added to polyamide provide excellent impact resistance but also causes lower mechanical strength.
The present invention additionally provides polyamide compositions containing the ionomer of the invention wherein said toughened compositions have improved impact resistance versus conventional ionomers and further provides an improved balance of properties of impact resistance and mechanical strength.